bug(mac): Searching for a Keyboard to download in Keyman for macOS gives no feedback when Internet is unavailable

If there is not connection to the Internet, when the user goes into the Keyman Configuration window and clicks Download Keyboard, they shouldn't just see a page with whatever their last search returned. Currently, that is what they see, but clicking any of the links there does nothing. Additionally, changing the language in the "Search by Language" box and clicking Search neither causes the page to change nor gives any indication that the API call failed. We need to catch that condition and inform the user that they do not have an Internet connection.